Project Summary Waterpipe tobacco is one of the most commonly used tobacco products among U.S. young adults. Waterpipe tobacco exposes users to high levels of carcinogens and other harmful toxicants, can lead to long-term addiction, and is associated with cancer and other negative health outcomes. Despite this evidence, young adults do not view waterpipe tobacco to be harmful or addictive, perceptions that contribute to the high prevalence of use. Waterpipe tobacco intervention research has focused on clinical cessation interventions among older, dependent users. These interventions likely have limited impact among young adults because their use of clinical cessation interventions is generally low and they do not view themselves to be in need of such cessation supports for waterpipe. Our research indicates public health messaging conveying the harms and addictiveness of waterpipe tobacco may be effective to shift young adult waterpipe tobacco users? perceptions and motivate behavior change. This study builds from our preliminary work and addresses a critical gap in this research area by rigorously examining the effects of messages conveying the harms and addictiveness of waterpipe tobacco using text and visual imagery and delivered by mobile phone multimedia messaging service (MMS) among young adults ages 18 to 30 who use waterpipe tobacco. Our approach leverages online crowdsourced data collection for message refinement and two-way, interactive mobile phone technology to engage with participants, deliver MMS messages containing text and visual imagery, and test the added effects of personally tailored MMS message content. Participants will complete baseline measures, receive basic message content on the harms and addictiveness of waterpipe tobacco use, and be randomized to three conditions: 1) a control condition receiving no additional messaging, 2) an untailored MMS condition, or 3) a tailored MMS condition. For a 1 month period, participants assigned to MMS conditions will receive text message prompts on two days each week delivered to their mobile phones via an interactive messaging system to assess waterpipe tobacco perceptions and behavior. After responding to prompts, participants will immediately receive MMS messages on the harms and addictiveness of waterpipe tobacco. For the tailored MMS condition, message content will be personalized to their responses to baseline measures and dynamically through the interactive text message exchanges during the intervention period. Waterpipe tobacco use and hypothesized mediators will be reassessed 1-, 3-, and 6-months post-baseline. Our study will generate critically needed data on the effects of mobile MMS messaging and tailored MMS message content to inform waterpipe tobacco use interventions. Our study directly addresses National Cancer Institute tobacco control research priorities, and now that waterpipe tobacco is regulated by the Food and Drug Administration (FDA) this study can guide the FDA?s efforts to educate the public about the harms of waterpipe tobacco.